Cisco IOS Voice Troubleshooting and Monitoring -- FXO Interfaces

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An FXO interface is used for trunk, or tie line, connections to a PSTN CO or to a PBX that does not support E&M signaling (when local telecommunications authority permits). This interface is of value for off-premises station applications. Figure: FXS and FXO Signaling Interfaces shows an FXS connection to a telephone and an FXO connection to the PSTN at the far side of a WAN.

Figure: Two-Port FXO Card Front Panel

Figure: Four-Port FXO Card Front Panel

RJ-21 Connectors on the High-Density Analog Telephony Network Module

The High-Density Analog Telephony network module supports an RJ-21 connector. This network module supports both FXS and FXO traffic. An illustration of the connector port is shown in Figure: High-Density Analog Telephony Network Module. Information about LEDs and pinouts can be found in the "Connecting High-Density Analog Telephony Network Modules to a Network" chapter of the Cisco Network Modules Hardware Installation Guide.

Figure: High-Density Analog Telephony Network Module

Shutdown Port

If the port is not working, be sure the port is not shut down. Enter the show voice port command with the voice port number that you are troubleshooting. The output will tell you:

If the voice port is up. If it is not, use the no shutdown command to make it active.

What parameter values have been set for the voice port, including default values (these values do not appear in the output from the 'show running-config' command). If these values do not match those of the telephony connection you are making, reconfigure the voice port.

Disabling a Port on a Multiple Port Card

If you shut down a port on a multiple-port card, you can disable all of the ports on that card. If only one port is bad and the others are working, in many cases you can disable the bad port and use the working ports until a replacement arrives. To disable a bad port, use one of the following methods:

On a Cisco universal gateway, such as the Cisco AS5350, Cisco AS5400, Cisco AS5800, and Cisco AS5850, busy out the port using the busyout command. This allows the port to be taken out of service without disrupting the Cisco IOS configuration. Refer to the product documentation for details:

FXO Disconnect Failure

When loop-starting signaling is used, an FXO interface looks like a phone to the switch that it is connecting to. The FXO interface closes the loop to indicate off hook. The switch always provides a battery so there is no disconnect supervision from the switch side. Because a switch expects a phone user or modem to hang up the phone when the call is terminated on either side, it also expects the FXO port on the router to hang up. However, the FXO port expects the switch to tell it when to hang up. Because the port relies on the switch, there is no guarantee that a near- or far-end FXO port will disconnect the call once either end of the call has hung up.

The most common symptoms of this problem are phones that continue to ring when the caller has cleared, or FXO ports that remain busy after the previous call should have been cleared.

The FXO Answer and Disconnect Supervision feature enables analog FXO ports to monitor call-progress tones and to monitor voice and fax transmissions returned from a PBX or from the PSTN.

Answer supervision can be accomplished in two ways: by detecting battery reversal, or by detecting voice, fax, or modem tones. If an FXO voice port is connected to the PSTN and battery reversal is supported, use the battery reversal method. Voice ports that do not support battery reversal must use the answer supervision method, in which answer supervision is triggered when the DSP detects voice, modem, or fax transmissions. Configuring answer supervision automatically enables disconnect supervision; however, you can configure disconnect supervision separately if answer supervision is not configured.

Disconnect supervision can be configured to detect call-progress tones sent by the PBX or PSTN (for example, busy, reorder, out-of-service, number-unavailable), or to detect any tone received (for example, busy tone or dial tone). When an incoming call ends, the DSP detects the associated call-progress tone, causing the analog FXO voice port to go on-hook.

This section provides solutions to problems that you might encounter when implementing the FXO Answer and Disconnect Supervision feature.

Typical problems with the answer supervision feature are as follows:

Call-progress tones such as ringback are not heard by the calling party.

If any call legs have IVR configured, ensure that the IVR version is 2.0.

Ringback timer is not initiated or ringback is not detected.

The wrong call-progress tone (cptone) command is configured on the voice port.

The wrong DTMF detection parameters are configured.

Custom call-progress tones are assigned to the voice port but ringback tone has not been configured; in this case, the default behavior is not to detect any ringback tones.

Answer supervision is not triggered.

Answer supervision-either by battery-reversal detection or by call-progress tone detection-is not configured on the voice-port in use.

Excessive delay before answer supervision is activated.

The level on the sensitivity parameter in the supervisory answer dualtone command is set too low. Configure the sensitivity for high.

If incorrect disconnect cause codes are reported, check the following:

The values configured for custom call-progress tones could be incorrect.

Overlapping detection frequencies might have been incorrectly specified in the voice class created by the voice class dualtone-detect-params command. For example if the freq-max-deviation parameteris configured to be 20 Hz, and the busy and reorder parameters are set forfrequencies 350 and 370 respectively, the voice port cannot detect the reorder tone, resulting in an incorrect disconnect cause code.

Note:

If the frequencies and cadences (including error deviations as defined in the voice class dualtone-detect-params command) are the same for multiple call-progress tones, the order of detection is as follows: busy, reorder, number-unobtainable, out-of-service, disconnect.

If calls are not billed correctly, it might be that answer supervision is not being triggered. For answer supervision to be triggered, voice, fax, or data tones originating at the called-party end must be detected.

Monitoring and Maintaining FXO Answer and Disconnect Supervision

To monitor the status of the FXO Answer and Disconnect Supervision feature, use the show voice port command, which causes the FXO voice port to be monitored. The following table illustrates the use of the show voice port command for monitoring voice port 1/1/0.

Command

Purpose

Router# show voice port 1/1/0

Shows a detailed status of the voice port. Under the heading "Voice card specific Info Follows:", the status of the FXO Answer and Disconnect Supervision feature is indicated by one of the following messages: "Answer Supervision is active"or "Answer Supervision is inactive".

Unbreakable Dial Tone

A common problem encountered in a VoIP network is being unable to break dial tone. The router puts a seizure on the local PBX but when digits are dialed, the dial tone stays. The calling party is unable to pass the DTMF tones or digits to the terminating device, resulting in callers being unable to dial the desired extension or interact with the device that needs DTMF tones, such as a voice mail or IVR application. Here are some possible causes of the problem:

DTMF tones not being sent.

DTMF tones not being understood.

DTMF tones too distorted to be understood.

Other signaling and cabling issues.

Make sure the dial type is set as DTMF on both the router and the PBX. The FXS port does not pass on the digits, therefore this setting is not available on an FXS port. However, this setting can be changed on FXO and E&M ports:

Troubleshooting Caller ID Problems

Several debugs can be used to troubleshoot the Caller ID feature on the routers. The voice port module (VPM) signaling debugs, such as the debug vpm signal command, track the standard debugs with Caller ID feature turned on. These debugs are analyzed from the perspective of the terminating router and its FXO port; the caller ID is sent from this end. The following example shows an FXO port receiving caller ID. In this example, the phone sends the caller ID to the FXO port.

In this example, everything was working fine and both Name and Number Display were properly delivered to the phone. In the two scenarios below, the calling number is missing in one case, and the name display is missing in the other.

Calling Number Lost, Name Delivered

In the following example, the calling number is lost, but the name is delivered: